Suspension type work platforms, also commonly referred to as access platforms, are well-known in the art. Such platforms are typically powered by a hoist at each end of the platform that raises and lowers the platform on an associated suspension wire at each end. The hoists are generally very simple machines including an electric induction motor, a gearbox, and a traction mechanism that grips the wire. Generally the electric motors are single-speed motors, however two-speed motors are available. Traditionally the motors incorporate across-the-line starters and therefore switch from off to full speed at the press of a button. The gearboxes reduce the motor speed resulting in a platform velocity generally ranging from 27 feet per minute (fpm) to 35 fpm. Therefore, the acceleration of the work platform from standing still to 27 fpm, or more, occurs essentially instantaneously and is jarring and dangerous, not only to the occupants but also the roof beams, or anchorage points.
Similarly, traditional systems offer no control over a powered deceleration of the work platform. This is particularly problematic when trying to stop the work platform at a particular elevation since the platform approaches the elevation at full speed and then stops instantaneously. This crude level of control offered by traditional systems results in repeated starting, stopping, and reversing, or “hunting,” before the desired elevation is obtained. Such repeated starts and stops not only prematurely wear the equipment, but are dangerous to the work platform occupants.
Additionally, the hoists used in suspended work platform systems are often several hundred feet from a power source making voltage drop through the conductors a concern that often results in motors overheating, premature failure, stalling, and the introduction of boost transformers. For instance, a typical window washing application may require that a work platform be suspended over five hundred feet from the location of the power source, which is typically at the top of the building. Such systems often require boost transformers located at the top of the building so that the voltage at the location of the hoist remains high enough to facilitate proper operation of the motor(s).
What has been missing in the art has been a system by which the users, employers, equipment manufacturers, or the hoist controls themselves can control the acceleration of the work platform. Further, a system in which the velocity can be adjustably limited depending on the particular working conditions is desired.